1. Field of the Invention
The present invention relates to a structure of a stator yoke of a stepping motor.
2. Related Art
In a claw-pole-type stepping motor, when an outer yoke is fixed to an inner yoke, a method such as press fitting, caulking, bonding and welding between an inside of the cup-shaped outer yoke and an outer circumferential portion of the inner yoke contacting the inside of the outer yoke, is used. However, the outer and inner yokes are produced by press working, so that a round portion is formed by bending at a cup-shaped opening side end portion which is a fixing portion for the outer yoke and a material flow is formed by punching at the outer circumference that is a fixing portion for the inner yoke. In this condition, when the fixing portions of the inner and outer yokes are contacted on the same plane, the contacting area is uneven according to conditions of the round portions caused by bending and material flow caused by punching. As a result, characteristics of the motors are not uniform.
As a solution to these problems, Japanese Patent Application, First Publication No. 2002-112522 discloses a technique described below. In this technique, a yoke is fixed by laser welding in addition to press fitting and bonding, and therefore leakage of a magnetic flux and destabilization of a magnetic circuit, which are caused by differences in strength of the fixing portion, are avoided, and unevenness in qualities of motors can be inhibited. Japanese Patent Application, First Publication No. 2007-143253 discloses a structure in which, focusing on a specific shape of a yoke, magnetic resistance is reduced by providing a reinforcing plate to a magnetic circuit for an enlargement of the magnetic circuit and characteristics of a motor can be improved.